1. Field of the Invention
The disclosure relates to the correction of mismatches on I and Q signals originating from signals broadcast by terrestrial channel.
The disclosure also applies in particular to the field of digital terrestrial television, for example as defined in the European DVB-T (Digital Video Broadcasting-Terrestrial) specification, or in the DVB-H (Digital Video Broadcasting-Handheld) specification.
The disclosure may also be applied to the field of digital broadcasting, as defined, for example, in the DAB (Digital Audio Broadcasting) standard.
The disclosure may equally be applied to the field of wireless local area networks, such as, for example, are defined in the IEEE 802.11 or Hiperlan/2 standards.
The disclosure relates in particular to demodulators and processing of the received broadcast signals.
2. Description of the Related Art
As a general rule, in high bit-rate communications, transmissions are limited among other things by the distortion of the signal during propagation. The data may be dispersed in time, thus creating inter-symbol interference.
Furthermore, a signal broadcast by terrestrial channel may be reflected on an obstacle during transmission. The obstacle may, for example, be a wall, a building or a relief element. The broadcast signal may also be subject to a refraction due to the index of a medium passed through, or even be diffracted against an obstacle. Consequently, the signal received by a receiver is the combination of a signal transmitted over a direct path from a sender and a multitude of attenuated and delayed signals originating from the different indirect paths.
The transfer function of such a channel may thus not be flat frequency-wise. Furthermore, the obstacles, the sender or the receiver may be mobile. The transfer function may thus change over time.
The use of an OFDM (Orthogonal Frequency Division Multiplexing) modulation is known. Transmission is handled by means of a frequency-division multiplexing of sub-carriers orthogonal to each other, separated by a guard interval. The modulation step involves an inverse Fourier transform and the demodulation step involves a fast Fourier transform (FFT). The OFDM modulation allows transmitting signals over a radio frequency channel with a relatively high reliability.
In particular, a COFDM (Coded OFDM) modulation may be used. The COFDM modulation allows providing a transmission that is relatively robust against the attenuations that may affect the sub-carriers.
A radio signal reception device comprises a tuner. The tuner allows replacing the signal received in the appropriate frequency band. The tuner may thus replace the signal received about an intermediate frequency, or even directly in the baseband. In the latter case, the tuner may be implemented in CMOS or BiCMOS technology. The tuner may thus have a relatively small size and consumption, which may be particularly interesting for DVB-H applications.
The tuner transforms the received signal into a signal in phase, denoted I (for In phase) and a signal in quadrature, denoted Q, respectively on an I channel and on a Q channel. The I signal and the Q signal are analogue. Mismatches between the I and Q channels may appear. The mismatches comprise phase defects, i.e., the phase shift between the vectors corresponding to the I signal and to the Q signal is not exactly 90°. The mismatches also comprise amplitude defects, i.e., the vectors corresponding to the I and Q signals have different lengths.
The patent application FR2853486 discloses a device comprising a baseband tuner, digitization means and a digital block. In addition to demodulation means, the digital block comprises correction means. These correction means are intended to correct the phase and amplitude mismatches of the I and Q channels. The patent application FR2853486 refers to known algorithms for correcting the phase and amplitude mismatches.
For example, an error is measured and a corrective phase shift to be introduced between the signals of the I and Q channels is calculated based on this error.
However, these algorithms have proved unsatisfactory when it comes to correcting mismatches on I and Q signals originating from signals broadcast by terrestrial channel.